Heretofore, a material comprising polyvinyl chloride resin (PVC resin) has been used as a wire coating material. PVC resin as a wire coating material has excellent insulating properties and is inexpensive. However, when the wire coating material comprising PVC resin is burnt after it is scrapped, it causes an environmental pollution problem such as the generation of a gas containing chlorine at the time of the disposal of the scrappage. In the field of transportation such as automobiles and trains, wire coating materials are desired to be light in weight and thin along with the weight reduction of a car body and the space saving of a wire all of which are aimed to save energy. Then, an olefin-based material is proposed as an environmentally friendly wire coating material (refer to Patent Document 1). Although the olefin-based material is free from an environmental pollution problem, very useful and inexpensive, an olefin-based material which is satisfactory in terms of mechanical strength and abrasion resistance for the reduction of the weight and thickness of a wire coating material is not obtained yet.
Meanwhile, a thermoplastic polyester resin out of polyester resins which are engineering plastic polymers has been used as the above material other than olefins. Since the thermoplastic polyester resin is excellent in heat resistance, abrasion resistance, electric properties, chemical resistance and moldability and has low absorptivity and excellent dimensional stability, it is used in a wide variety of fields such as automobiles, electric and electronic appliances, insulating materials and OA equipment. For example, since it has the above characteristic properties, it is disclosed that it can reduce the weight and thickness of wire coating while it retain abrasion resistance (refer to Patent Document 2).
However, the further improvement of the heat resistance and mechanical properties of an insulating material for wires has been desired in recent years. Then, to improve heat resistance, there is proposed a material obtained by adding an epoxy group-containing additive to a thermoplastic polyester resin (refer to Reference Document 3). However, satisfactory heat deterioration resistance is not obtained yet.
To further improve heat resistance, there is also proposed a material obtained by adding a carbodiimide compound to a thermoplastic polyester resin (refer to Patent Document 4). However, when the softening temperature of the carbodiimide compound is low, that is, the molecular weight of the polycarbodiimide is low, there occurs a problem that heat deterioration resistance degrades due to a small steric barrier around the carbodiimide group. Further, there is another problem that the polycarbodiimide having a low softening temperature is adhered to a port for injecting raw materials at the time of producing a composition, thereby making it impossible to obtain industrially advantageous productivity.
There is proposed use of a composition having excellent heat resistance, abrasion resistance, flame retardancy and hydrolysis resistance, which is prepared by adding a polyester block copolymer, an inorganic porous filler and a carbodiimide compound to a thermoplastic polyester resin, as an insulated wire (refer to Patent Document 5). However, the further improvement of insulating properties is desired.
Further, there is proposed use of a composition having heat resistance, flame retardancy, hydrolysis resistance and abrasion resistance, capable of achieving low smoke-producing properties and containing no halogen compound, which is obtained by adding a polyester block copolymer, magnesium hydroxide, an inorganic porous filler and a carbodiimide compound to a thermoplastic polyester resin, as an insulated wire (refer to Patent Document 6). However, since insulating properties are degraded by magnesium hydroxide which is added to achieve low smoke-producing properties, the improvement of the insulating properties is desired.    (Patent Document 1) JP-A 2009-249390    (Patent Document 2) JP-A 2002-343141    (Patent Document 3) JP-A 9-263685    (Patent Document 4) JP-A 2007-211123    (Patent Document 5) JP-A 2010-100724    (Patent Document 6) JP-A 2010-121112